The present invention relates generally to an apparatus and method for spot welding an upper coated sheet including a semi-circular hole or notch formed at an edge to a lower coated sheet at the location of the semi-circular hole or notch.
Spot welding type process have been employed to secure overlapping sheets of material together. In one variant, a circular hole is punched in an upper sheet at the desired location of the weld, and the upper sheet is positioned over a lower sheet. The two sheets are then spot welded together at the location of the circular hole. The sheets are commonly made of zinc coated sheet steel or aluminum or magnesium alloys with tenacious surface oxide films.
The circular hole is generally punched away from the edge of the upper sheet because it is difficult to punch circular holes near the edge without deforming or wrinkling the upper sheet. Additionally, as the circular hole is punched closer to the edge of the upper sheet, the material between an edge of the circular hole and the edge of the upper sheet is more likely to melt during welding. This can create a very brittle joint that has variable strength properties and a poor cosmetic appearance.
The distance between an edge of the circular hole and the edge of the upper sheet should ideally be at least equal to the diameter of the circular hole. If the sheets are made of aluminum, magnesium or other materials having a high thermal conductivity, the distance between the edge of the semi-circular hole or notch and the edge of the upper sheet should ideally be at least 1.5 times the diameter of the semi-circular hole or notch to prevent over-melting of the upper sheet. However, if the upper sheet is a narrow flange, it may not be possible to locate the circular hole away from the edge of the upper sheet sufficiently far to create optimal conditions.
Hence, there is a need in the art for an apparatus and method for spot welding an upper sheet to a lower sheet that prevents deformation and wrinkling of the upper sheet, that produces a spot weld having a good cosmetic appearance, and overcomes other problems of the prior art.